In the present specification, reference is made to the following prior art of conventional pulse laser devices.    [1] A. Giesen et al. in “IEEE Journal of Electronics” vol. 33, 2007, p. 598;    [2] J. aus der Au et al. in “Opt. Lett.” vol. 25, 2000, p. 859;    [3] C. R. E. Baer et al. in “Opt. Lett.” vol. 35, 2010, p. 2302-2304;    [4] S. V. Marchese et al. in “Opt. Express” vol. 16, 2008, p. 6397-6407;    [5] D. Bauer et al. in “Advanced Solid-State Photonics”, OSA Technical Digest (CD) (Optical Society of America, 2011), paper ATuC2.    [6] T. Südmeyer et al. in “Appl. Phys. B” vol. 97, 2009, p. 281-295;    [7] C. J. Saraceno et al. in “Conference Paper: The European Conference on Lasers and Electro-Optics (CLEO/Europe)” Munich, Germany, May 22, 2011;    [8] M. Tokurakawa et al. in “Opt. Lett.” vol. 33, 2008, p. 1380-1382;    [9] C. Hönninger et al. in “Appl. Phys. B” vol. 69, 1999, p. 3;    [10] S. Uemura et al. in “Jpn. J. Appl. Phys.” vol. 50, p. 010201;    [11] V. Magni et al. in “J. Opt. Soc. Am. A” vol. 4, 1987, p. 1962-1969;    [12] B. Henrich et al. in “Opt. Comm.” vol. 135, 1997, p. 300-304;    [13] G. P. A. Malcolm et al. in “Opt. Lett.” vol. 16, 1991, p. 1967;    [14] Walter Koechner, Textbook “Solid state laser engineering,” 6th rev. and updated ed. Springer 2010;    [15] S. G. Lukishova et al. in “Quantum Electronics” vol. 26, 1996, p. 1014;    [16] U.S. Pat. No. 6,363,090;    [17] E. P. Ippen et al. in “Appl. Phys. Lett.” vol 21, 1972, p. 348; and    [18] O. Pronin et al. in “Opt. Express” vol. 19, 2011, p. 10232-10240
Thin disk technology for laser resonators overcame long standing milestone of achieving high average power directly from the laser. Multi-kW levels are obtainable directly from the laser with one disk head [1] in CW multimode, and more than 100 W in average power is available in fundamental mode operation. Due to the thermal management of the thin gain disk medium and as a consequence of reduced thermal lensing of the material, excellent beam quality and high power operation are possible simultaneously. This feature made the thin disk concept attractive for femtosecond lasers since about 2000 [2]. Gain media suitable for femtosecond thin disk operation are described in [6]. The most common gain media is Yb:YAG having benefits in terms of low quantum defect, high gain cross section, broad absorption line at 940 nm, high thermal conductivity, thermomechanical strength and availability in large sizes with good optical quality. Some other Yb doped materials have been used as well, like Yb tung-states: Yb:KYW, Yb:KLuW. Yb sesquioxides Yb:Lu2O3, Yb:LuScO3, Yb: (Sc, Y, Lu)2O3 and Yb borates: YB:YCOB. These materials are attractive because of their broader emission bandwidth and therefore potentially shorter achievable pulses.
Power scaling experiments resulted in an average output power of 140 W obtained directly from the oscillator [3] and 10 μJ pulse energies from the cavity in which the disk is a one of the folding mirrors [4]. Moreover, 30 μJ output pulses have been obtained in the multipass cavity geometry [5]. However, in prior art it has been emphasized that pulse duration from Yb:YAG thin disk lasers is limited to about 700 fs [3, 6]. In [6] this limitation is related to the higher saturated gain and the reduced gain bandwidth due to a high inversion level at high power operation. The highest average power of ˜140 W is reported from an Yb:Lu2O3 based thin disk oscillator with 735 fs pulses. However the emission bandwidth of this material should support pulses as short as 100 fs or even below as it has been demonstrated in [8]. So far the short pulses of 194 fs are achieved from a Yb:LuScO3 thin disk laser with relatively low output power of 9.5 W [7]. Sub-100-fs-pulses have also been demonstrated from Yb: YAG gain medium by [10]. Nevertheless, spectral filtering had been applied and the wavelength was shifted from it's gain maximum at 1030 nm towards 1060 nm.
An early technique, which has been proposed historically far before the development of thin disk femtosecond lasers, includes the generation of ultra-short laser pulses by mode-locking of cw dye lasers [17]. Contrary to thin disk laser, the mode locking of dye lasers relied on the fast saturation dynamics of both the gain dye and saturable absorber dye, it does not need the initiation of the mode locking near the stability edge and cavity designs have small thermal load and small beam diameters over the cavity length. Femtosecond solid state thin disk laser relies on solution mode locking in the negative or positive dispersion regime. Moreover, Kerr lens mode locking includes very complex self focusing dynamics as well as complex cavity design. The complexity is caused by the analysis of the cavity behaviour near the stability edges which is necessary for reliable initiation of Kerr lens mode locking. Therefore cavity design and initiation of mode locking is much more complex task here compared with the dye laser techniques.
Thin disk femtosecond lasers can be operated on the basis of mode-locking using a Semiconductor Saturable Absorber Mirror (SESAM). The SESAM can be arranged as an end mirror in a cavity including concave resonator mirrors as schematically illustrated in FIG. 12 (prior art [2], see also [3]). According to FIG. 12, the conventional laser device 100′ includes a laser resonator 10′ with a gain disc medium 11′ and a SESAM 18′. A resonator section 13′ is provided for shaping the circulating electric field coupled into the gain disc medium 11′. The resonator section 13′ is made of three concave mirrors, which allow the setting of a large mode size in the gain disc medium 11′. One of the three concave mirrors is a folding mirror which simultaneously influences the mode size in the SESAM 18′. Until now the shortest pulse duration generated from an SESAM mode locked Yb:YAG oscillator is 340 fs with 170 mW average power [9].
SESAMs have a number of benefits in terms of reduction of thermal lensing, insensitivity to the cavity alignment, and easy implementation in a resonator cavity by substitution of one of the flat mirrors. On the other hand, SESAMs have a number of drawbacks relating to Q-switching instabilities, low damage threshold, two photon absorption, thermal lensing resulting from saturable and non-saturable losses, finite relaxation time and limited supported bandwidth. In particular, the damage threshold of the semiconductor is lower than that of glass. Therefore the damage threshold of the SESAM sets a limitation in maximum achievable pulse energies and minimum achievable pulse durations inside the laser cavity. Furthermore, thermal lensing limits the average achievable power from the oscillator. Damage of the SESAM can also be partially caused by the heating of the device.
Power scaling in thin disk laser geometries is possible by increasing the mode area on the disk medium in proportion to the pump power (and by keeping the peak pump power at the same level). By utilizing this principle power scaling was successfully demonstrated in SESAM mode locked thin disk oscillators. However, both the thin disk pumped at high average power and the SESAM having saturable and non-saturable losses exhibit thermal lensing effects. As it was shown by Magni [11] one thermal lens inside the cavity results in two stability zones for operating the laser. Zone I is less and Zone II is much more sensitive to misalignment. The second stability zone corresponds larger beam sizes inside the cavity. Moreover enlarging the beam size in the cavity leads to shrinking of the stability zones. The width of the stability zones depends on the beam size as ˜1/w2 (w-beam waist in the disk). These features introduce more restrictions and make designing the cavity a more complex task. Evidently, including a second thermal lens (SESAM) in a cavity will make designing the cavity for fundamental mode operation even more complicated and narrow the stability zones even further.
Furthermore, thin disk femtosecond lasers can create laser pulses on the basis of mode-locking with a Kerr medium as schematically illustrated in FIG. 13 (Kerr lens mode-locking laser, KLM laser, see prior art [10], [12], [13], [16]). The conventional laser device 100′ of FIG. 13 comprises a laser resonator 10′ including a gain disc medium 11′ and a mode-locking Kerr medium 12′. Furthermore, the laser resonator 10′ includes two curved concave resonator mirrors, which span a resonator section 13′ including the Kerr medium 12′. The circulating electric field in the laser resonator 10′ coupled into the Kerr medium 12′ is shaped in the resonator section 13′ by the effect of the curved mirrors. The Kerr medium 12′ is located in the focus formed by the curved mirrors and the gain disc medium 11′ is used as a folding cavity mirror. Accordingly, the resonator section 13′ simultaneously shapes the beam size in the gain disc medium 11′.
Due to the following disadvantages and restrictions, the resonator design of FIG. 13 is barely applicable in practice. Typically small beam sizes inside the cavity lead to the high risk of damaging of optical components. These effects are especially pronounced near the edges of the stability zones of the laser resonator. The beam waist in the gain disk medium 11′ is strongly dependent on the beam waist in the Kerr medium 12′ (the bigger the waist in the disk the smaller the waist in a Kerr medium). Accordingly, for larger spot sizes in the gain disk medium 11′ such laser resonator 10′ has to be completely redesigned and reoptimized. For the case of symmetric X shape cavity beam waist in the flat mirror wd related to the beam waist in the Kerr medium wk by ratio wd˜f/wk. Furthermore, cavity stability zones are strongly influenced by thermal lensing in thin disk and dispersive optics. Finally, the cavity length is strongly dependent on the large mode sizes in a cavity and Kerr medium.
At the moment, thin disk oscillators are the most promising way of achieving high powers and high energy pulses simultaneously from a compact table top system. But until now no thin disk laser generating emission-bandwidth-limited pulses or even beyond the emission-bandwidth-limit at high power level has been realized yet.
Generally, further cavity geometries are known in the field of solid state lasers. A laser resonator 10′ can have e.g. a telescopic geometry or a concave-convex geometry as schematically illustrated in FIGS. 14A and 14B (prior art [14], page 248 and pages 219/221), resp. The telescope section 13′ of FIG. 14A can be used to enlarge the spot size in the bulky gain medium 11′. The resonator providing the “concave-convex” geometry (FIG. 14B) is adapted for achieving a large mode size inside the laser resonator 10′ as well. It is also known to use resonators combining concave-convex and telescopic geometries. However, the telescopic geometry and the concave-convex geometry have not yet been used in the field of mode-locked thin disk laser resonators.